1. Technical Field
The present invention relates to electronic memory devices, and more particularly, to systems and methods for detecting disturbed memory cells of semiconductor memory devices.
2. Related Art
Semiconductor memory devices are well known and commonly found in a variety of electronic devices. Most semiconductor memory devices can be classified as either volatile or nonvolatile. A volatile memory device requires power in order to maintain the stored data, while a nonvolatile memory device can retain stored data even in the absence of power.
A well known type of nonvolatile memory is flash memory. A typical flash memory includes a memory cell array in which memory cells are arranged in rows and columns. Each memory cell includes a floating gate field-effect transistor. The logic state of a memory cell depends on the threshold voltage of the transistor, which in turn depends on the number of electrons in the floating gate of the transistor. Electrons in the floating gate partially cancel the electric field from the control gate, thereby modifying the threshold voltage of the transistor. Thus, the logic state of a flash memory state can be controlled by controlling the number of electrons in the floating gate of the transistor.
A flash memory cell can be programmed and erased in order to write respective logic states to the memory cell. The program and erase operations correspond to write operations for respective logic states, which correspond to respective threshold voltages. For convenience, the threshold voltages will simply be referred to as high and low threshold voltages, with it being understood that the high threshold voltage is relatively higher than the low threshold voltage by some detectable voltage margin. The number of electrons stored in the floating gate of a memory cell transistor can be altered by applying a strong electric field between the control gate and at least one of the source, drain, and substrate of the field-effect transistor in order to remove or accumulate electrons in the floating gate. An “erase” operation can be an operation where electrons are removed from the floating gate, thereby reducing the threshold voltage of the memory cell transistor to the low threshold voltage. A “program” operation can be an operation where electrons are accumulated in the floating gate, thereby increasing the threshold voltage of the memory cell transistor to the high threshold voltage. Since erased and programmed memory cells can be readily distinguished due to the difference in threshold voltages, the erased and programmed memory cells can be used to represent different logic states. For example, an erased memory cell can be representative of a logic state “1,” while a programmed memory cell can be representative of a logic state “0.”
Thus, each memory cell can store one or more bits of data. For example, dual bit memory devices allow storage of two bits of data in a single memory cell, one bit being stored in a trapping layer proximate to the source region and the other being stored in a trapping layer proximate to the drain region. The presence or absence of negative changes in one area of the trapping layer may represent the status of the corresponding bit.
In multi-bit memory devices that utilize an ONO stacked layer to store charges, the charge added or removed during the programming and erase operations should be confined to the respective source and drain regions of a cell. However, in reality, the buildup and removal of charges in one of the source and drain regions may affect the charge in the other region, thus changing the reading, programming and erase characteristics of the cell over time. Eventually, the buildup of charges changes the threshold voltages for determining the state of the respective bits in a cell. The shift in threshold voltages due to the interactions between the two regions of a cell is generally known as bit disturb. Additionally, charge loss after repeated cycling of the memory cell also causes a shift in the threshold voltages, especially the program threshold voltage. The change in threshold voltages, which occurs as a result of bit disturb and/or charge loss, prevents a memory device from correctly sensing the state of each bit of the cells.
The state of each bit of a multi-bit cell, including a programmed state or in an erased state, is determined by comparing the value from a memory cell (e.g., the voltage level of a data bit) with a reference value. Generally, reference cells are pre-programmed and set to an erased state during initial manufacturing so that a stable reference value may be provided. When the value from the memory cell is greater than the reference value, the memory cell is determined to be in an erased state. On the other hand, when the value from the memory cell is less than the reference value, the memory cell is determined to be in a programmed state. FIG. 1 shows a diagram illustrating sensing margin of a memory device. The line designated as PIDEAL represents the ideal program threshold voltage of a memory cell over time. The line designated as PDISTURBED represents an example of the change in the program threshold voltage of the memory cell over time considering effects such as bit disturb and charge loss. The line designated as EIDEAL represents the ideal erase threshold voltage of the memory cell over time. The line designated as EDISTURBED represents an example of the change in the erase threshold voltages of the memory cell over time considering effects such as bit disturb and charge loss. The line designated as REF represents the reference value provided by a reference cell of a memory device in the prior art, which remains a constant value over time as shown in FIG. 1. Referring to FIG. 1, due to effects such as bit disturb and/or charge loss effect, the sensing margin between the reference value REF and the program threshold voltages PDISTURBED of the memory cell may decrease over time, for example from margin M1 to margin M2, which may increase the possibility of sensing errors and reduce the reliability of memory devices.
In addition, floating gate memory cells can be susceptible to certain failure modes over the lifetime of the memory device. Some such failure modes involve disturbs. Disturbs occur when reading, erasing, or programming one memory cell causes an unwanted alteration of data at another memory cell. For example, one such type of disturb is known as the “erase disturb.” An erase disturb occurs whenever the data stored in a memory cell is unintentionally changed while an erasing operation is being performed on another memory cell. This can occur, for example, whenever the two memory cells share a common well. Relatively high voltages are used in order to perform an erase operation. Under certain conditions, the high erase voltage used to erase a first memory cell may unintentionally affect a second memory cell, for example causing a transfer of electrons from the floating gate of the second memory cell and thereby decreasing the threshold voltage of the second memory cell. Such unintended changes can cause unwanted data alterations and read errors to occur.
Thus, there is thus a general need in the art for effective methods of detecting and correcting disturbed memory cells in non-volatile memory devices.